Which formula does this calculator use?

It uses the low-pressure Spitzglass formula referenced by NFPA 54 and IPC Appendix A for fuel gas under 1.5 psi. The equation relates flow, inside diameter, length, and specific gravity, and is solved here for the pressure drop in inches of water column.

What flow value should I enter in SCFH?

Enter the connected appliance demand carried by that segment. If you know the load in BTU per hour, divide by the gas heating value — roughly 1030 BTU per cubic foot for natural gas and 2516 for propane — to get SCFH. The tool also shows the heat input back-calculated from your flow.

Why is 0.3 in WC the usual target?

Most residential and light-commercial low-pressure systems are designed so the total drop from the meter or regulator to the farthest appliance does not exceed 0.3 in WC, preserving appliance manifold pressure. A 0.5 in WC allowance is sometimes used where the supply pressure has margin.

Does this include fittings and valves?

No — it sizes one straight segment. Add the equivalent length of elbows, tees, and valves to the measured pipe length before entering it, or check each segment and sum the drops along the longest run to the most remote outlet.

Can I use this for medium or high-pressure gas?

No. The low-pressure Spitzglass form assumes the pressure drop is small relative to atmospheric. Systems above about 1.5 psi require the high-pressure sizing equations and a different compressibility treatment, so use the appropriate NFPA 54 method for those.

What is the Fuel Gas Pressure Drop Calculator (NFPA 54)?

Uses the NFPA 54 / IPC Appendix A low-pressure Spitzglass formula to compute pressure drop in a natural gas or propane pipe segment from flow in SCFH, Schedule 40 diameter, length, and gas specific gravity, then checks the 0.3 or 0.5 in WC limit. It runs free in your browser on Gera Tools, with nothing uploaded.

Fuel Gas Pressure Drop Calculator (NFPA 54)

Name: Fuel Gas Pressure Drop Calculator (NFPA 54)
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Get one useful tool a week

Like this tool? Enter your email and we'll send you one genuinely useful Gera tool a week — plus a link to come back to this one. No spam, one-click unsubscribe any time.

Keeping gas pressure where it belongs

A fuel gas pipe must deliver enough gas to every appliance without dropping the pressure below what the burners need. NFPA 54 and the IPC let designers size low-pressure systems so the total pressure loss stays within a small allowance — commonly 0.3 inches of water column (in WC). This calculator computes the drop through a pipe segment so you can confirm a run is large enough before it goes in the wall.

How it works — the Spitzglass formula

For low-pressure gas (under about 1.5 psi) the code references the Spitzglass formula. Its low-pressure form relates flow to the square root of diameter to the fifth power times pressure drop, divided by length and specific gravity, with a diameter-dependent friction factor Cr = 1 / sqrt(1 + 3.6/D + 0.03·D). Solving for the pressure drop gives:

H = ( Q / (3550 · Cr) )^2 · ( Cg · L ) / D^5

where H is the drop in inches of water column, Q is flow in SCFH, D is the actual inside diameter in inches, Cg is gas specific gravity (0.60 for natural gas, 1.52 for propane), and L is length in feet. The tool uses Schedule 40 steel inside diameters and back-calculates the heat input from your flow using the gas heating value (approximately 1,030 BTU/scf for natural gas and 2,516 BTU/scf for propane).

Worked example

Say you have a furnace drawing 100,000 BTU/h and a water heater drawing 40,000 BTU/h on the same branch — total load 140,000 BTU/h. Converting to SCFH for natural gas: 140,000 ÷ 1,030 ≈ 136 SCFH. The branch is 30 ft of 3/4-inch Schedule 40 pipe (inside diameter 0.824 in).

Plugging into the Spitzglass formula with Cg = 0.60:

Cr = 1 / sqrt(1 + 3.6/0.824 + 0.03×0.824) ≈ 0.424
H = (136 / (3550 × 0.424))^2 × (0.60 × 30) / 0.824^5
This works out to roughly 0.22 in WC — comfortably under the 0.3 in WC limit.

Extend that branch to 60 ft and the drop roughly doubles to about 0.44 in WC, exceeding the allowance. The fix is to upsize to 1-inch pipe (ID 1.049 in), which drops the loss well below 0.1 in WC.

Key design rules

Pressure drop scales with the square of flow and linearly with length. Doubling the flow quadruples the drop; doubling the run length doubles it.
Add equivalent length for fittings. An elbow typically adds 1–2 ft of equivalent length per nominal diameter; a tee can add more. Include these in the length you enter.
Check the longest path, not each segment in isolation. The total drop from the meter to the farthest appliance is what must stay within the 0.3 or 0.5 in WC allowance.
Manifold pressure matters. Natural gas appliances typically need about 3.5 in WC at the appliance manifold; propane appliances need about 11 in WC. If supply pressure minus the calculated drop still clears these values, the sizing is adequate.
Propane runs through smaller pipes. Because propane has a higher heating value per cubic foot, the SCFH flow is much lower for the same BTU/h load, so propane systems often size down one nominal pipe size compared to natural gas for the same appliance load.